Recently, in the optical pickup apparatus, the wavelength-shortening of the laser light source used as the light source for reproducing of the information recorded in an optical disk or recording of the information in the optical disk is advanced. For example, a laser light source of wavelength 405 nm such as a blue-violet semiconductor laser, or a blue-violet SHG laser which conducts the wavelength conversion of the infrared semiconductor laser by using the second harmonic wave generation is putting to a practical use.
Using these blue violet laser light sources enables an objective lens with the same numerical aperture (NA) as DVD (Digital Versatile Disk) to record the information of 15-20 GB in an optical disk of diameter 12 cm, and enables an objective lens whose NA is increased to 0.85 to record the information of 23-25 GB in the optical disk of diameter 12 cm. Hereinafter, in the present specification, the optical disk and photo-magnetic disk for which the blue violet laser light source is used, are generally referred as “high density optical disk”.
Hereupon, only by saying that the information can be adequately recorded and/or reproduced for such a high density optical disk, it is difficult to be said that a value as a product of the optical disk player and/or recorder is enough. In the present time, DVD or CD (Compact Disc) in which various information are recorded is put in a market. When the actuality is based on, by only a case where the information can be recorded and/or reproduced for the high density optical disk, it is insufficient, and for example, a fact that the information can be adequately recorded and/or reproduced also for a user-own DVD or CD, introduces to a fact that a commercial value as the optical disk player and/or recorder is increased. For such a background, it is desirable that the optical pickup apparatus mounted in the optical disk player and/or recorder for the high density optical disk has a performance by which the information can be adequately recorded and/or reproduced while the compatibility is being kept with also any one of 3 kinds of optical disks of the high density optical disk and DVD, and CD.
As a method by which the information can be adequately recorded and/or reproduced while the compatibility is being kept with also any one of the high density optical disk and DVD, furthermore, CD, a method by which an optical system for the high density optical disk and an optical system for DVD or CD are selectively switched corresponding to the recording density of the optical disk for which the information is recorded and/or reproduced, can be considered. However, because a plurality of optical systems are necessary for the method, it is disadvantageous for down-sizing, further, the cost is increased.
Accordingly, for the purpose to intend that the structure of the optical pickup apparatus is simplified and the cost is reduced, it can be said that even in the optical pickup apparatus having the compatibility, it is preferable that the common optical system for the high density optical disk and for DVD or CD is used and the number of parts structuring the optical pickup apparatus are reduced at most. Further, it is most advantageous that the common objective optical system arranged toward the optical disk is used with each other, in the simplification of the structure of the optical pickup apparatus and the cost reduction.
Hereupon, in the case where it is intended to realize the compatibility by using the common objective optical system in the optical pickup apparatus, the light source wavelengths or the protective substrate thicknesses used for respective optical disks are different. Therefore, it is necessary that any idea for forming a light converged spot whose aberration is finely corrected on the information recording surface of the optical disk.
As a mode of the aberration correction, it is considered that an angle of the light flux incident on the objective optical element is changed. According to such a mode of the aberration correction, when a degree of the divergence of the light flux incident on the objective optical element is changed, the aberration correction corresponding to the using optical disk can be conducted (refer to Patent Document 1).
[Patent Document 1] Tokkai No. 2001-60336
However, when the objective optical element is arranged on the common optical path, a design that each of light fluxes of all wavelengths λ1, λ2 and λ3 enters into the objective optical element as an almost infinite parallel light flux for recording and/or reproducing of information, is preferable because the off-axis light performance is increased. Although, in a technology written in Patent Document 1, because the divergent light flux is incident at the time of the information recording and/or reproducing of CD, there is a possibility that that the off-axis light performance is worsened. That is, as larger the degree of the divergence of the incident light flux on the objective optical element is, there is a possibility that the coma is generated larger when the lens shift is conducted at the time of the tracking.